The invention relates generally to a zoom lens and more particularly to a reverse telephoto zoom lens suitable for use in a compact digital camera.
Compact, inexpensive cameras and recently, digital cameras, are currently very popular among consumers. One of the more desirable features of these cameras is a zoom lens. This has challenged lens and camera designers to develop more compact zoom lenses that are camera compatible in terms of camera size and mechanical simplicity while still providing acceptable imaging quality. Digital cameras present a further challenge. The electronic image capture media typically used in digital cameras, such as CMOS or CCD arrays, often employ microlenslet arrays to enhance light collection by the pixels and to suppress aliasing. To work effectively, however, the lens system directing the light to these arrays should ideally be telecentric or nearly so because the effectiveness of the arrays is a function of the angle of incidence of the incoming bundle of light. By telecentric, it is meant that the light impinging the microlenslet array is substantially parallel to the optical axis and strikes the array at a very shallow impingement angle.
Three-moving-group zoom lenses and afocal zoom lenses with a fixed positive lens group are able to provide a high degree of telecentricity but at the expense of numerous elements including costly aspheric components, loss of compactness, and complicated mechanical zoom drive systems.
Historically, two-group (negative, positive) zooming retrofocus or reverse telephoto lens systems were used in 35 mm SLR cameras. In spite of their bulkiness, they provided the long back focal lengths necessary to accommodate the movable mirror in the SLR camera design. This same bulkiness attribute, however, detracted from consideration of this type of lens system for use in compact digital cameras. Nevertheless, the comparatively smaller format of a digital camera CCD compared to the 35 mm film format enables the use of such a properly designed lens system, such as that set forth herein, in a compact digital camera. Moreover, reverse telephoto lens systems advantageously address the telecentricity issue because of their inherent long back focal lengths and corresponding xe2x80x9csoftxe2x80x9d impingement angles.
As mentioned above, telecentric lens systems comprising an afocal zoom lens and a fixed positive rear element whose front focus falls at the aperture stop for all zoom positions generally have an excessive front vertex-to-image plane distance (FVIP) resulting in a long, bulky lens assembly. In addition, the FVIP and the baseline length (i.e., the absolute minimum length of the lens at all focal lengths) are, respectively, functions of the powers of the front and rear lens groups. Typically, in moving between the telephoto and wide angle positions, the front zooming group in these lens systems undergoes parabolic motion relative to the fixed rear group thus increasing the mechanical complexity of the camera. At the same time, rear lens group power is limited in order to ease aberration correction. Film based cameras that use a two-moving-group (positive, negative) lens system often create very large angles of impingement of the ray bundle on the image plane in wide angle mode and thus are undesirable for digital camera use.
Zoom lenses comprising a first divergent zooming lens unit and a second convergent zooming lens unit are known. The second lens unit of such zoom lenses typically comprises three or more positive power lens elements with at least one negative power lens element sandwiched between two of these positive power lens elements (i.e., plus, minus, plus configuration). Such zoom lenses are described in U.S. Pat. Nos. 4,155,629; 4,299,452; 5,552,937 and 5,563,739. U.S. Pat. Nos. 4,999,007 discloses a zoom lens having a first divergent zooming lens unit and a second convergent zooming lens unit. The second lens unit of this zoom lens has an overall positive power but contains only one positive power lens element and two negative power lens elements. Thus the positive power lens element has to be very strong. This feature, in turn, is a source for optical aberrations and affects the overall image quality.
Accordingly, the inventor has recognized a need for a less costly, less mechanically and optically complex, and more compact zoom lens suitable for use particularly in a compact digital camera, which provides excellent image quality, which is environmentally stable, and which addresses the problems identified above.
The invention is generally directed to a reverse telephoto zoom lens and more particularly to such a zoom lens delivering near telecentric performance that is especially suitable for use in a compact digital camera.
According to an embodiment of the invention, a zoom lens comprises a first lens group that includes a first lens component which is positioned closer to an object than any other lens component of the zoom lens, and a second lens group that includes a last lens component which is positioned closer to an image plane than any other lens component of the zoom lens, wherein said last lens component has a concave surface facing the image plane. In an aspect of this embodiment, the first lens group has overall negative power and the second lens group has overall positive optical power. In another aspect, the last lens has a convex surface opposite the concave surface. In a further aspect of this embodiment, the first lens group includes a lens having both positive and negative power in respective radial zones of the lens. In another aspect, the second lens group includes a biconvex lens adjacent the last lens.
In another embodiment, a zoom lens, comprises a first lens group including a first lens component positioned closer to an object than any other lens component, and a second lens group including a last lens component positioned closer to an image plane than any other lens component, wherein the first lens group includes a lens component having both positive and negative power in respective radial zones of the lens. In an aspect of this embodiment, the last lens has a concave surface facing the image plane.
According to another embodiment of the invention, a zoom lens comprises, in order from an object side, a first zooming lens unit having divergent optical power and a second zooming lens unit having convergent optical power (i.e., a negative, positive lens system). The first zooming lens unit includes a first, negative power lens element, a second lens element, and a third, positive power lens element. At least one of these lens elements has an aspherical surface. The second zooming lens unit includes, in order, a fourth, positive power lens element, a fifth, positive power lens element and a sixth, negative power lens element having a concave surface towards an image forming side of the system. Zooming is performed by changing a distance between the first lens unit and the second lens unit.
The above, and other objects, advantages and novel features of the present invention will become more apparent from the accompanying detailed description thereof when considered in conjunction with the following drawings.